Optical recording media are characterized in that they do not contact a recording or reading head so that they do not undergo wear or deterioration; that high density recording is possible with a light beam of condensed laser rays, etc.; that recording and reading can be carried out at a high speed; and that the access time is short.
Further, recording on such optical recording media can be effected by converting information to be recorded to electric time series signals and scanning the media with a laser beam modulated in accordance with the signals. This recording system is advantageous in that recorded images can be obtained in a real time.
The above-described optical recording media include photomagnetic recording media using MnBi type polycrystalline materials or rare earth element-transition metal type amorphous alloys, e.g., Gd-Co, Gd-Tb-Fe, etc. In this recording system, recording is carried out by both heating with laser rays and external application of a magnetic field, and reproduction is carried out by making use of a difference in rotational direction of a vibration plane of light, arising from a difference in magnetic direction. In particular, vertical photomagnetic recording media in which magnetic lines are perpendicular to the recording media can be used for high density recording. However, these photomagnetic recording media have a disadvantage of poor sensitivity in reproduction and very low S/N ratios, which cause errors in reading out, giving rise to a serious bar to practical application.
The optical recording media further include media comprising a thin film of a metal, e.g., Te, Bi, As-Te-Se, etc., in which a condensed laser beam is irradiated on the metal thin film to locally vaporize the metal to form pits, and reproduction is effected by reading the presence of the pits.
These recording media, however, involve problems such that laser rays of high power are required; that control on pit shape is so difficult as to result in high noise level; and the like.
Additional optical recording media include those utilizing transformation of Te-TeO thin film from an amorphous state to a crystalline state, and those utilizing a blackening phenomenon attributable to structural changes of an Se-S type thin film between metastable states within an amorphous phase thereof.
However, the Te type media has problems of toxicity and of insufficient contrast between recorded areas and unrecorded areas. Further, the Se-S type thin film shows low recording sensitivity, and its absorption edge is in a relatively short wavelength region. In the vicinity of such an absorption edge, light absorption is weak, and recording sensitivity at longer wavelengths is particularly poor.
Under these circumstances, it has keenly been demanded to develop recording media having improved recording sensitivity, improved S/N ratios by high density recording and increased optical contrast between recorded areas and unrecorded areas, and freedom from toxicity.